1. Field of the Invention
This invention relates to high temperature fuel cell stacks, particularly molten alkali metal carbonates fuel cell stacks using thin metal separator plates. The separator plates of this invention provide improved electrolyte containment and may provide make-up electrolyte during cell operation.
2. Description of Related Art
Generally, fuel cell electrical output units are comprised of a stacked multiplicity of individual cells separated by inert or bi-polar electronically conductive ferrous metal separator plates. Individual cells are sandwiched together and secured into a single stacked unit to achieve desired fuel cell energy output. Each individual cell generally includes an anode and cathode electrode, a common electrolyte tile, and a fuel and oxidant gas source. Both fuel and oxidant gases are introduced through manifolds to their respective reactant chambers between the separator plate and the electrolyte tile. The area of contact between the electrolyte and other cell components to maintain separation of the fuel and oxidant gases and prevent and/or minimize gas leakage is known as the wet seal. A major factor attributing to premature fuel cell failure is corrosion and fatigue in the wet seal area. This failure is hastened by corrosive electrolyte contact at high temperatures and high thermal stresses resulting from large temperature variations during thermal cycling of the cell causing weakening of the structure through intracrystalline and transcrystalline cracking. Such failures permit undesired fuel and/or oxidant gas crossover and overboard gas leakage which interrupts the intended oxidation and reduction reactions thereby causing breakdown and eventual stoppage of cell current generation. Under high temperature fuel cell operating conditions, in the range of about 500.degree. to 700.degree. C., molten carbonate electrolytes are very corrosive to ferrous metals which, due to strength requirements, are necessary for fuel cell housings and separator plates. The high temperature operation of stacks of molten carbonate fuel cells increases both the corrosion and thermal stress problems in the wet seal area, especially when the thermal coefficients of expansion of adjacent materials are different.
Commercially viable molten carbonate fuel cell stacks may contain up to about 600 individual cells each having a planar area in the order of ten square feet. In stacking such individual cells, separator plates separate the individual cells with fuel and oxidant each being introduced between a set of separator plates, the fuel being introduced between one face of a separator plate and the anode side of an electrolyte matrix and oxidant being introduced between the other face of the separator plate and the cathode side of a second electrolyte matrix. The problems of manifolding and sealing become more severe when larger number of cells and larger planar areas are used in the cell stack. When greater number of cells are used, the electrical potential driving the carbonate in the seal area along the height of the stack increases, and when the planar area of the cell increases, the linear tolerances of each component and the side alignment of each component becomes extremely difficult to maintain in order to maintain the mating surface sealed between the manifold/manifold gasket/and cell stack. Cell stacks containing 600 cells can be approximately 10 feet tall presenting serious problems of required stiffness of external manifolds and the application of a clamping force required to force the manifold onto the cell stack. Due to the thermal gradients between cell assembly and cell operating conditions, differential thermal expansions, and the necessary strength of materials used for the manifolds, close tolerances and very difficult engineering problems are presented.
Conventionally, stacks of individual molten carbonate fuel cells have been constructed with spacer strips around the periphery of a separator plate to form wet seals and to provide intake and exhaust manifolds. Various means of sealing in the environment of the high temperature fuel cell wet seal area are disclosed in U.S. Pat. No. 4,579,788 teaching the wet seal strips are fabricated utilizing powder metallurgy techniques; U.S. Pat. No. 3,723,186 teaching the electrolyte itself is comprised of inert materials in regions around its periphery to establish an inert peripheral seal between the electrolyte and frame or housing; U.S. Pat. No. 4,160,067 teaching deposition of inert materials onto or impregnated into the fuel cell housing or separator in wet seal areas; U.S. Pat. No. 3,867,206 teaching a wet seal between electrolyte-saturated matrix and electrolyte saturated peripheral edge of the electrodes; U.S. Pat. No. 4,761,348 teaching peripheral rails of gas impermeable material to provide a gas sealing function to isolate the anode and cathode from the oxidant and fuel gases, respectively; U.S. Pat. No. 4,329,403 teaching graded electrolyte composition for more gradual transition in the coefficient of thermal expansion in passing from the electrodes to the inner electrolyte region; and U.S. Pat. No. 3,514,333 teaching housing of alkali metal carbonate electrolytes in high temperature fuel cells by use of a thin aluminum sealing gasket.
Gas sealing of a phosphoric acid fuel cell, which operates at about 150.degree. to 220.degree. C., by filling the pores of a porous material periphery of the cell constituents with silicon carbide and/or silicon nitride is taught by U.S. Pat. No. 4,781,727; and by impregnating interstitial spaces in substrate plate edge is taught by U.S. Pat. Nos. 4,786,568 and 4,824,739. The solution of sealing and corrosion problems encountered in low temperature electrolytic cells, such as bonding granular inert material with polytetrafluorethylene as taught by U.S. Pat. No. 4,259,389 gaskets of polyethylene as taught by U.S. Pat. No. 3,012,086; and "O" ring seals taught by U.S. Pat. No. 3,589,941 for internal manifolding of fuel only are not suitable for high temperature molten carbonate fuel cells.
U.S. Pat. No. 4,910,101 teaches fuel cell stacks having exterior extensions on the separator plates to form receivers for surplus electrolyte in a gas discharge manifold to the exterior of the fuel cell stack and provides means for return of recovered electrolyte to the same cell from which it leaked without substantial pressure loss. The receivers on the separator plates also provides a method for addition of electrolyte to the operating fuel cell.
U.S. Pat. Nos. 4,963,442 and 5,045,413 teach fully internal manifolded fuel cell stacks wherein the electrolytes and separator plates extend to the edge of the fuel cell stack and form a peripheral wet seal by the separator plate having a flattened wet seal structure extending from each face of the separator plate to contact the electrolytes completely around their periphery to form a separator plate/electrolyte wet seal under fuel cell operating conditions. The electrolytes and separator plates each have a plurality of aligned perforations, the perforations in the separator plates each being surrounded by a flattened manifold wet seal structure extending from each face of the separator plate to contact the electrolytes to form a separator plate/electrolyte wet seal under fuel cell operating conditions thereby providing a plurality of manifolds extending through the fuel cell stack for fully internal manifolding of fuel and oxidant gases to and from each unit fuel cell in the fuel cell stack. U.S. Pat. No. 5,077,148 teaches a fully internal manifolded and internal reformed fuel cell stack having separator plate/electrolyte seals similar to those taught by U.S. Pat. Nos. 4,963,442 and 5,045,413 and having interspersed along its axis a plurality of reforming chambers formed by adjacent separator plates to provide fully internal manifolding of reactant gas and steam to product gas from each reformer unit in the fuel cell stack.